For example, scooter type motorcycles mount thereon a belt type continuous variable transmission, of which transmission gear ratio can be adjusted in a stepless manner according to a traveling state. Belt type continuous variable transmissions comprise a primary sheave, a secondary sheave, and a belt. The belt is entrained between the primary sheave and the secondary sheave in an endless manner.
The primary sheave comprises a stationary sheave body and a movable sheave body, which are opposed to each other, and receives torque transmitted from an engine to be rotated. The movable sheave body is slidable in a direction toward and away from the stationary sheave body, and a belt groove is defined between the movable sheave body and the stationary sheave body to permit the belt to be entrained therein.
Further, the primary sheave comprises a cam plate and a plurality of roller weights. The cam plate is opposed to the movable sheave body. The roller weights are interposed between the cam plate and the movable sheave body so as to rotate together with the movable sheave body. The roller weights are aligned at intervals circumferentially of the movable sheave body, and movable radially of the movable sheave body.
The secondary sheave interlocks with a rear wheel of a motorcycle through a speed reducer. The secondary sheave comprises a stationary sheave body and a movable sheave body, which are opposed to each other. The movable sheave body is slidable in a direction toward and away from the stationary sheave body, and a belt groove is defined between the movable sheave body and the stationary sheave body to permit the belt to be entrained therein. The movable sheave body is biased by a spring in a direction, in which the belt groove is decreased in width.
When the primary sheave is increased in rotating speed, the roller weights move radially outwardly of the movable sheave body according to centrifugal forces generated upon rotation of the movable sheave body. Such movements cause the movable sheave body to be pushed by the roller weights to slide toward the stationary sheave body. Therefore, the belt groove on the primary sheave is decreased in width, so that the belt interposed between the stationary sheave body and the movable sheave body is pushed out radially outwardly of the primary sheave. Accordingly, a diameter, at which the belt is entrained about the primary sheave, is increased.
In contrast, with the secondary sheave, the belt is pulled toward a center of rotation of the secondary sheave. Thereby, the movable sheave body slides against the bias of the spring in a direction away from the stationary sheave body. As a result, the belt groove is enlarged in width, and a diameter, at which the belt is entrained about the secondary sheave, is decreased. Therefore, the belt type continuous variable transmission is decreased in transmission gear ratio. The transmission gear ratio becomes minimum when a diameter, at which the belt is entrained about the primary sheave, becomes maximum.
With conventional belt type continuous variable transmissions, a minimum transmission gear ratio is determined by restricting positions of roller weights relative to a primary sheave. Stated specifically, a movable sheave body of the primary sheave comprises a plurality of stoppers that project toward an outer periphery of a cam plate. The stoppers come into contact with outer peripheral surfaces of the roller weights when the movable sheave body is slid to a position, in which a belt groove is made smallest in width. Such contact restricts movements of the roller weights caused by centrifugal forces, and thus a width of the belt groove and a diameter, at which the belt is entrained about the primary sheave, are determined to obtain a minimum transmission gear ratio. For example, JP-A-2001-248698 discloses a belt type continuous variable transmission provided with a primary sheave, which comprises such stoppers.
With the belt type continuous variable transmission disclosed in the patent document, roller weights are pushed against outer peripheries of a movable sheave body and a cam plate in that operating state, in which a transmission gear ratio becomes minimum. Generally, roller weights are made of a softer material than a movable sheave body and a cam plate. Therefore, when fresh roller weights are repeatedly pushed against the movable sheave body and the cam plate, those portions of outer peripheral surfaces of the roller weights, which contact with the movable sheave body and the cam plate, begin wear.
When the roller weights have worn, the movable sheave body shifts in a direction toward the cam plate. In other words, the movable sheave body cannot be pushed toward a stationary sheave body by an amount, by which the roller weights have worn, so that a belt groove on a primary sheave is increased in width. Accordingly, a diameter, at which the belt is entrained about the primary sheave, changes in a direction, in which a transmission gear ratio is increased, so that it is not possible to obtain a predetermined minimum transmission gear ratio.
FIG. 24 discloses a situation, in which speed ratio changes at a minimum transmission gear ratio in a conventional belt type continuous variable transmission. As apparent from FIG. 24, at a point of time when the travel distance of a motorcycle is 0 and roller weights are fresh, an actual minimum transmission gear ratio R1 of the belt type continuous variable transmission is maintained at a predetermined value R2. The minimum transmission gear ratio R1 changes in a direction of increasing speed ratio with the lapse of operating time. Further, the minimum transmission gear ratio R1 becomes stable at a point of time when pressures generated on contact portions of the roller weights and a movable sheave body and on contact portions of the roller weights and a cam plate reach certain values as the roller weights proceeds in the course of wear.
Accordingly, with conventional belt type continuous variable transmissions, it is not possible to avoid a change in speed ratio in a direction, in which a minimum transmission gear ratio is increased, caused by wear of roller weights. As a result, there is caused a problem of an increase in engine speed and a decrease in traveling speed of a motorcycle.